1. Field of the Invention
The invention generally relates to a full duplex communication system, and more particularly, to an echo cancellation device for a full duplex communication system.
2. Description of the Prior Art
As technology advances, network usage is more and more popular. The requirement of network bandwidth is increasing and the transmission speed of data packets of Ethernet has risen from 10/100 Mps to 1 Gbps.
Please refer to FIG. 1, which is a block diagram of a 1 Gbps fast Ethernet system. In the 1 Gbps Ethernet system, each port has 4 channels 100 where each channel has a transceiver 102 and a line interface 116 electrically coupled to a twisted line 118. The transceiver 102 has a transmitter 104 and a receiver 106, wherein the transmitter 104 has a digital-to-analog converter (DAC) 108 to convert signals into analog form for transmitting to a far-end network device via the line interface 116 and the twisted line 118, and the receiver 106 has an analog front end (AFE) circuit for processing the received signal from the line interface 116 and an analog-to-digital converter (ADC) 114 for converting the processed signals into digital signals and then sending to the after circuits. The fast Ethernet and the far-end network device both simultaneously utilize the four channels where each channel simultaneously performs the transmitting and receiving operations. As a result, the fast Ethernet system is a full duplex communication system.
Each channel of the fast Ethernet system simultaneously performs the transmitting and receiving operations. When the channel is transmitting, the signals received from the channel are affected by the transmission and this phenomenon is known as echo impairment. In order to reduce echo impairment, an echo cancellation device 110 and an echo cancellation resistor Rp are usually disposed in the transmitter 104 as illustrated in FIG. 1. The echo cancellation device 100 is usually a digital-to-analog converter (DAC) to output a cancellation signal that is similar to the output signal from the DAC 108 so the cancellation signal can cancel the adverse effects on the receiver 106 by the transmitted signals to achieve echo cancellation.
Please refer to FIG. 2, which is an equivalent circuit of the fast Ethernet device in FIG. 1. The circuit equivalence of the DAC 108 and the echo cancellation device 110 of the transmitter 104 are current sources Id and Ic respectively. For the receiver 106 to achieve echo cancellation, the output of the current sources Ic and Id must cancel the adverse effects caused by the transmitter 106.
Please refer to FIG. 3, which is a model of the equivalent circuit in FIG. 2. Zo is the effective output impedance which in FIG. 2 includes a matching resistor Rm for matching impedance and an effective resistor Rc of the channel. Vo is the output signal which is the transmitted signal from the transmitter 104 and is also the received signal from the receiver 106. From the circuit in FIG. 3, we can draw the following formula:
                    Vi        =                              -                          Zi              ⁡                              [                                  IdZo                  +                                                            (                                              Zo                        +                        Rp                                            )                                        ⁢                    Ic                                                  ]                                                          Rp            +            Zi            +            Zo                                              (        1        )            in order to cancel echo, set vi=0, which satisfies:IdZo+(Zo+Rp)Ic=0   (2)therefore the relationship between Ic and Id is:
                    Ic        =                                            -              Zo                                      Rp              +              Zo                                ⁢          Id                                    (        3        )            to accomplish echo cancellation.
The disadvantages of conventional echo cancellation device is that the effective output impedance Zo is seen as a pure load resistor Re, where the resistance characteristics are based on the matching resistor Rm and the effective resistor Rc. Besides the matching resistor Rm and the effective resistor Rc of the channel, the effective output impedance is also affected by the parasitic capacitance Ce that is unavoidable during operation of the circuit. If the effective output impedance is seen purely as a load resistor Re, echo cancellation cannot be effectively reduced to the lowest.